The invention relates to a valve drive for a cam-operated valve of an internal combustion engine, in which a closing force is applied to the valve against the opening direction of the valve by a valve spring, with a hydraulic force application device, with which a force can be applied directly or indirectly onto the valve against the direction of the closing force, in that a piston of the force application device is moved relative to a cylinder of the force application device by introducing hydraulic fluid into the pressure chamber formed by the piston and the cylinder in a displacement direction, wherein the piston can be moved relative to the cylinder from a first end position to a second end position.
Valve drives of this type are known in the state of the art, for example, from DE 101 56 309 A1 and from U.S. Pat. No. 4,796,573. They are used to generate additional valve lifting in addition to the opening lift of the valve that is dependent on the shape of the cam of a camshaft. For this purpose, a force application device is pressurized with hydraulic fluid in such a way that the valve lifting is, to a large extent, variable.
In DE 102 42 866 A1, which also belongs to this class, such a variable valve drive is provided, such that the valve lifting caused by the cams of the camshaft can be minimized by a control valve by shutting off hydraulic fluid from the control chamber of the force application device, whereby the control chamber can be connected to hydraulic fluid at high pressure.
The valve timing device known from EP 0 196 441 B1 has a valve piston, which has a stepped section in the form of an annular radial shoulder on one end. Through a special configuration of the valve piston, during the shut-off process, thus when compressed fluid from the working chamber of the force application device is shut off and therefore when the valve piston returns, an annular gap in a stepped and continuously tapering configuration is produced, whereby a pressure can be established, which generates end position damping of the valve piston.
Although an essentially variable influence on the valve lifting is already possible with the known valve drives, wherein damping of the movement of the force application device can also be realized in the end position, the known systems have a few disadvantages.
The targeted path-controlled braking of the piston of the force application device is not possible relative to the cylinder for a few solutions. Instead, as, for example, in U.S. Pat. No. 4,796,573, pressurization with hydraulic fluid is necessary for braking the piston, wherein the dynamics of the braking process are produced from the hydraulic behavior of the hydraulic elements used there.
Furthermore, in some of the known solutions, there is a relatively slow acceleration of the piston from the damping end position, which is disadvantageous.
The stepped pistons also known for targeted braking of the piston cause considerable production problems from time to time or have a complicated overall structure for the force application device as a result, which makes the systems costly.
If maximum stroke limiting through hydraulic shutoff is used, such force application devices have the disadvantage that the shutoff is burdened with losses, whereby the efficiency of the device is decreased.